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The soft tissue artefact (STA) is a common problem when using extracorporal 3D-markers for kinematic measurements. Only STA for the scapula from the upper girdle has been evaluated in an invasive study [1]. In contrast, a non invasive MRI based evaluation was used in the presented study to quantify the influence of STA on an extracorporal 3D-marker attached to the upper arm. The upper arm of 6 volunteers without shoulder instability was scanned in an open MRI scanner in 5 static positions of the arm elevation in the scapula plane. A MRI detectable 3D-marker, a method to determine a humeral coordinate system from its 3D-image and a unique analysis of the marker motion with respect to the underlying humerus were used in order to quantify the amount and direction of STA. In order to quantify the humeral coordinate system determination error without the influence of STA three evaluation experiments were performed using virtual rotations of a 3D-image of the humerus, physical rotations of a humerus model and 4 Operators which segmented a 3D-image of a volunteer’s shoulder.
The results showed that the STA influence on the marker position with respect to the humerus was significantly bigger in all directions than the translational and rotational errors caused by the humeral coordinate system determination from MRI scans. STA induced humeral translational artefacts of the extracorporal 3D-marker position of almost 4 mm along the humeral long axis and 8 mm along the humeral transverse axes, while the rotational artefacts were approximately 5 ° about the long axis and 2.5 ° about the transverse axes. Furthermore the direction of STA rotations and translations showed large interindividual variations. Therefore, an individual STA correction is necessary because under pathologic conditions the glenohumeral translation during the arm elevation begins after 5 mm [2].
[1]: Karduna et al. (2001). J Biomech Eng 123:184-190.
[2]: Kelkar et al. (2001). J Shoulder Elbow Surg 10:73-84.